1990 steering problem...Help

hiya, I just bought a 1990 cherokee, I knew of the problem with the steering when I bought it, here's what I have
the power steering is intermittent, sometimes it works and other times it does'nt, seems to sometimes work turning to the left and not the right, I figured it was the pump, but after I got the truck home the pump appears to be new, can anyone give me a clue as to what my problem is, I was also told that there was a power steering filter on these? if so where is that located